Unsaturated polyester resins have hitherto been used as coating compositions for woodworking, molded articles, sealing materials and adhesives because they have good gloss and desirable thick appearance, and also impart beautiful finishing appearance. Also the unsaturated polyester resin is used in combination with a styrene monomer, as a crosslinkable unsaturated monomer, in view of the curability and physical properties. However, in view of environmental problems, a social trend of regulating the use of the styrene monomer, that can cause problems such volatility, odor, and toxicity, has recently increased. When using the styrene monomer as coating materials such as coating compositions, there arise problems in that grain depression of a coating film is caused by volatilization of the styrene monomer during curing of the coating film, resulting in variability in quality.
To cope with these problems, (1) a method of reducing the content of a styrene monomer by decreasing the molecular weight of an unsaturated polyester resin, (2) a method of suppressing volatilization of a styrene monomer by adding wax to a unsaturated polyester resin composition containing a crosslinkable unsaturated monomer, and (3) a method of using a crosslinkable unsaturated monomer having low volatility in place of a styrene monomer are proposed. However, the methods (1) and (2) do not constitute a radical solution to the above problems because of the use of the styrene monomer, while those having satisfactory curability and drying characteristics, especially thin film drying characteristics in the coating film, have not been obtained by the method (3).